1. Field of the Invention
The invention relates to real time methods for sizing or adjusting edges and corners of grayscale pixel maps for input into raster or shaped beam pattern generators typical of radiant beam lithography writing tools.
2. Description of the Prior Art
Using computational techniques (algorithms) and computers to manipulate grayscale pixel maps of patterns and images is a standard, well known practice in many fields of graphics and data analysis. A pixel is typically understood as the smallest identifiable element or area composing a picture, pattern or image. A pixel map simply expresses the location of each pixel composing the picture, pattern or image in context of a two dimensional coordinate system. A pixel's gray level defines its relative intensity to the maximal level allowed in the mapping.
Radiant energy beam lithography systems are commonly used in integrated circuit production processes to print patterns or masks onto semiconductor wafers. In such systems, pixel maps of polygons, e.g., triangles trapezoids and rectangles, are created where each pixel is quantified and expressed or printed onto a mask or wafer surface by the radiant energy beam. The dose or level of exposure the pixel is determined by a grayscale assigned to the corresponding pixel, typically 0 to a maximum, e.g., 16, where 0 corresponds to 0-dose or black, and 16 corresponds to 16-dose or white. The intervening levels correspond to ascending levels of gray toward white.
In printing systems, images and characters are represented in grayscale pixel maps where each pixel corresponds to a unit area or dot, and the grayscale assigned to each pixel determines the level or densities to be expressed or printed at each corresponding unit area. For color images, complementary color pixel maps are generated and combined to reproduce an image. [See U.S. Pat. No. 6,021,255 Hayashi et al. & U.S. Pat. No 6,141,065 Furuki, et al.] In additive or radiant systems such as computer monitors, Red, Blue and Green (RGB) pixel maps are created and combined to project an image. In subtractive or reflective systems such as poster images, Cyan, Magenta, Yellow, blacK (CMYK) pixel maps or dots are printed and combined onto a surface to reflect an image.
Once created, digitally expressed pixel maps can be enlarged or magnified, reduced or de-magnified, and/or distorted or morphed using algorithms and computers. However, when pixel resolution is in nanometer (micro inch) ranges, the number of pixels per macroscopic unit area is astronomical. Time and memory requirements for processing and then storing such high resolution pixel maps can be quite substantial even with advanced data compression schemes and high cycle [MHz & GHz] CPU processors.
In applications where issues such as depth and color are not of concern, (e.g., radiant energy beam lithography applications) the most significant elements of the mask or pattern expressed in the pixel maps are the boundaries between the 0 dose (black) regions and the maximum dose (white) regions. Such boundaries are expressed in levels of grayscale. Yet as skilled and even unskilled manipulators/editors of pixel maps have experienced, a Cartesian array of rectangular pixels cannot continuously express an inclined boundary, i.e., a boundary not aligned with the Cartesian coordinates of the expressing system. While grayscale allows some smoothing (anti-aliasing) of inclined boundaries, such boundaries remain rough, meaning the expressed boundary varies somewhat regularly between limits. Such boundary roughness can cause problems particularly in very large-scale integrated circuits (VLSI circuits) where feature sizes range below the 150 nm scale.
Photo and other lithography systems present a host of boundary or edge effects including scattering, wavelength, and effects of the components directing the writing radiant beams. In addition there are boundary effects arising from: (i) the properties of the mediums in/onto which the masks or pixel maps are written or expressed, (e.g. refraction and substrate reflectivity); and (ii) subsequent processing of the exposed mask or printed pixel map, (e.g., etch bias).
In other words, skilled semiconductor mask designers are confronted with a dilemma of having to create VLSI circuitry patterns or masks for each particular lithography tool, each particular wafer composition, and each expected post exposure wafer processing scheme. An alternative would be to design and store a digitized pixel map of an idealized (master) mask and then use a real time computational process or procedure to modify or ‘size’ the master mask on the fly to meet anticipated parameters imposed or expected of the particular lithography tool, the particular wafer composition and/or the particular post exposure wafer processing scheme.
A primary criterion for determining the efficacy, hence desirability of any particular computer implemented algorithm or technique for modifying a pixel map is time. Algorithms that limit or minimize the number of operations that must performed by the computer to effect a acceptable change are preferable to those that effect an accurate or correct change but are expensive time wise and/or computationally.